At present, most electronic devices use an AC adapter to provide necessary power or to charge electronic devices. After the AC adapter is plugged into the wall-outlet in a household or office, the AC adapter converts AC power to DC power, which electronic products can make use of.
For ordinary portable electronic devices, such as a mobile phone or personal digital assistant (PDA), which are normally powered by a battery, the AC adapter is used in certain condition. As the outlet may be located at a position on the wall not convenient for the user to plug, after the user finishes charging the portable electronic device, the user normally removes the portable electronic device from the AC adapter instead of removing the AC adapter from the wall-wallet.
However, the AC adapter left plugged in the wall-outlet still consumes power and results in power waste despite the AC adapter is no more charging the portable electronic device. According to a practical experiment for the measurement of the AC adapter of a mobile phone, there are a current of 1.5 milli-amperes flowing through the AC adapter when the AC adapter of the mobile phone is at full capacity. After the mobile is unplugged, there are still a current of 0.93 milli-amperes, which are about 60% of the one at full capacity, flowing through the AC adapter. Nowadays, environmental conservation and power saving are highly valued. Thus, it is an important subject in the industry to develop an AC adapter which reduces the power consumption when being plugged in a wall-outlet.